One major concern in the production of high temperature superconductor layers or films is the achievement of a current carrying capacity sufficiently high for practical use.
Intrinsic to high temperature superconductor (htsc) material is the electrical anisotropy with high electric conductivity in a,b-direction and only low electrical conductivity in c-direction. In addition, grain boundaries with more than 4°-5° misalignment angle act as weak links. The critical current density for grain boundaries above that misalignment angle decreases exponentially as a function of that angle.
Consequently, single crystal high temperature superconductor material has good current carrying capacity in a,b-direction whereas in polycrystalline material a drastic drop of the current carrying capacity is observed.
It has been shown that the reason for this drop is misalignment of the individual grains or crystals constituting the polycrystalline film in a,b-direction. For solving the problem of a,b-axis misalignment intensive studies have been made.
There are well known techniques for epitaxially growing single crystal films using a substrate having a crystallographic lattice parameter close to that of the high temperature superconductor film to be grown.
In these cases also the substrate itself is single crystalline.
Here, the crystal orientation of the substrate is transferred to the growing film. That is, the substrate serves as a template for inducing the desired orientation of the film.
However, suitable single crystalline substrates are very expensive and the surface area is only limited so that large scale production of high temperature superconductor films with long length is not possible and, consequently, the commercial importance of these techniques is only minor.
Similar to single crystal films also polycrystalline films when grown on a polycrystalline template of a material with similar lattice parameters adopt the crystalline orientation of the template material and, thus, can be oriented. That is, when using a polycrystalline template with grains of suitable orientation the orientation can be transferred to a polycrystalline film grown on the template provided that the lattice parameters of the template are sufficiently similar, i.e. matching, to those of the film to be grown.
Generally, high temperature superconductors are typically defined as having a critical temperature (tc) above the temperature of liquid nitrogen. Examples of such high temperature superconductors are superconducting ceramic oxides belonging to the bismuth-strontium-calcium-copper-oxide family (BSCCO), yttrium-barium-copper-oxide family (YBCO) and thalium-barium-calcium-copper-oxide family (TBCCO-family).
All of these superconducters are cuprates and crystallise with perovskite structure.
The crystal structure is characterised in having copper oxide planes which define the current flow path.
In the production of coated conductors YBCO-based high temperatures superconductor materials, such as Y1Ba2Cu3O (YBCO-123), are commonly used nowadays.
Summarising, to be suitable as a template for growing high temperature superconductor films with improved current carrying capacity a polycrystalline film must have    1. good a, b-alignment as well as    2. a crystal structure with lattice parameters matching to those of the high temperature superconductor material.
Alignment in a,b-direction is also referred to as biaxial texture.
“Good a, b-alignment” means that the misalignment angle between the a-axis and b-axis, respectively, of different grains is as small as possible; in particular grains grown with 90° tilted orientation, the so-called a,b-grains, should be absent.
A polycrystalline film with only very low misalignment angles is referred to to have a “sharp” biaxial texture.
Techniques for biaxially texturing buffer layers to be suitable as templates used in the production of coated conductors are generally known. In particular, there are two major techniques.    1. In the “ion beam assisted deposition” (IBAD) an oxide layer is deposited on a polycrystalline metal tape by a vacuum deposition technique like pulsed laser deposition (PLD) or evaporation. During deposition the growing film is hit by a monoenergetic Ar-ion beam under a specific angle for removing grains with undesired orientation from the film. By this technique in particular materials such as yttrium-stabilized zirconia (YSZ), MgO and Gd2Zr2O7 are biaxially textured.    2. There is known the “inclined substrate deposition” technique (ISD). Also in this technique an oxide layer is deposited on a polycrystalline metal tape by a vacuum deposition technique such as PLD or evaporation. Deposition is done such that the trajectories of impinging vaporized metal species have a certain angle with the substrate normal. This technique is usually applied for biaxially texturing MgO.
Though these techniques allow the use of non-oriented substrates, they generally suffer from either only low production rate or poor texture quality. Further they are vacuum processes with the need of specific equipment with demanding production conditions.
A specific example for a buffer layer used in coated conductors with YBCO-123 superconductor films is yttrium-stabilized zirconia (YSZ) obtained by ion beam assisted deposition as referred to above.
YBCO as well as YSZ crystallize with a cubic face centred lattice. As reported in EP 1 178 129 the distance between an atom located at a corner and an atom located at the centre of a face of the cubic lattice is 0.363 nm for YSZ and for YBCO-123 0.381 nm. Though the distance in YSZ is considered to be fairly close to those of YBCO-123, nevertheless, an intermediate layer of Y2O3 is required with a distance of 0.375 nm for bridging the difference in lattice size between YSZ and YBCO-123.
For avoiding the need of a second layer with closer lattice match EP 1 178 129 suggests using a material of pyrochlore type structure as buffer layer said material having a general formula of AZrO or AHfO with A being selected from Y, Yb, Tm, Er, Ho, Dy, Eu, Gd, Sm, Nd, Pr, Ce and La, and having a cubic lattice such as YBCO.
In the general formula the relative proportion of A and Zr and Hf, respectively, is 1:1 with a possible variation of from 0.1:0.9 to 0.9 to 0.1 provided that the cubic crystal system is maintained.
As specific examples of suitable materials with pyrochlore type structure reference is made to compounds of the formula A2Zr2O7 and A2Hf2O7 with A as defined above and with the distance between the atoms closest together decreasing from 0.381 nm (La2Zr2O7, La2Hf2O7) to 0.366 nm (Yb2Zr2O7, Yb2Hf2O7).
Due to the identical distance of 0.381 nm with YBCO the La compounds are considered to be the most promising candidates for serving as template in transferring biaxial texture to growing YBCO film.
Consequently EP 1 178 129 teaches that for epitaxial growth identical lattice parameters are preferable.
As is the case with YSZ buffer layer in EP 1 178 129 biaxial texturing of the buffer layer is carried out by ion beam assisted deposition with the drawbacks as set out above.
In view of the above, further improvement for obtaining buffer layers with suitably sharp texture is highly desired in order to grow superconductor films with biaxially texture of good quality to achieve superior current carrying capacities.
Moreover, a method for attaining a coated conductor in a practically long length with a high temperature superconductor film with sharp biaxial texture and, thus, high current carrying capacity, in a simple and effective manner is needed allowing production of such long length coated conductors in large scale with economically reasonable costs.
In particular there is a desire to have a method for obtaining suitably textured templates not requiring vacuum based techniques for obtaining the template.